I have an iMac desktop computer, Intel based, with Snow Leopard running on it. When I am running iPhoto the small photos appear in the right pane as they should. However, when I double click on one of the photos to enlarge it, many of them will show a black screen with a white circle and a black exclamation point in the white circle. When I click it again, it returns to the smaller size and is visible. If I use the slider, the pictures will get bigger but when I double click on them the black screen with the exclamation point appears. Again, when I click the black screen, the smaller picture re-appears. This phenomenon does not happen on all of the pictures, just some. Also, the photos that this occurs on are not copies of the originals. When I highlite the photo to edit it and click on edit the screen also goes black. Does anyone have a clue as to what is causing this and how I might fix it?